Evaporative fuel treatment apparatus for internal combustion engine

ABSTRACT

An evaporative fuel treatment apparatus for an internal combustion engine includes: a communication portion that communicates a plurality of branch passages with one another at positions downstream of the plurality of throttle valves; a purge passage that introduces purge gas, containing evaporative fuel, to the communication portion; an air supply passage that flows dilution air, which is used to dilute the evaporative fuel, into the purge passage; a first flow rate changing portion that is provided in the air supply passage and that is able to change the inflow of the dilution air; and a control unit that controls the first flow rate changing portion.

INCORPORATION BY REFERENCE

The disclosure of Japanese Patent Application No. 2008-009003 filed on Jan. 18, 2008 including the specification, drawings and abstract is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a technical field of an evaporative fuel treatment apparatus for an internal combustion engine, which treats evaporative fuel generated in a fuel tank.

2. Description of the Related Art

An evaporative fuel treatment apparatus for an internal combustion engine of this type is, for example, described in Japanese Utility Model Publication No. 3-53009 (JP-Y-3-53009). JP-Y-3-53009 describes an evaporative fuel treatment apparatus that is applied to an internal combustion engine in which a throttle valve is arranged in each independent intake pipe provided for each cylinder. In this evaporative fuel treatment apparatus, the independent intake pipes are in fluid communication with all the cylinders via a communication pipe, which is common to all the cylinders, at positions downstream of the throttle valves, and then the communication pipe is connected to a purge pipe that introduces purge gas containing evaporative fuel generated in a fuel tank. In addition, Japanese Patent Application Publication No. 7-189820 (JP-A-7-189820) describes an evaporative fuel treatment apparatus in which an assist air type fuel injection valve is arranged in each independent intake passage provided for each cylinder, a purge passage is connected to an assist air passage that feeds air to the fuel injection valve, and then purge gas is supplied together with assist air through the fuel injection valve to each independent intake passage. Moreover, a technique is suggested, which, in an internal combustion engine in which a throttle valve is arranged in each of the above described independent intake pipes, a throttle portion (opening area is invariable) is provided in an air supply passage.

On the other hand, for example, Japanese Patent Application Publication No. 08-144818 (JP-A-08-144818) describes a technique that, in a V-engine that has an independent intake system and exhaust system in each bank, a purge passage is switched to stop the inflow of purge gas bank by bank. In addition, Japanese Patent Application Publication No. 5-79359 (JP-A-5-79359) describes a multi-cylinder internal combustion engine that includes an independent throttle valve provided in an intake passage of each cylinder and a second throttle valve provided in a common intake passage upstream of the independent throttle valves. Moreover, a technique is suggested, which a pressure in the intake pipe is calculated to efficiently cause purge gas to flow into the internal combustion engine.

These apparatuses introduce purge gas to positions downstream of the throttle valves through the communication pipe; however, this may cause an abrupt change in air-fuel ratio because the directly introduced evaporative fuel has a high concentration. Specifically, in a single throttle valve type in which a single throttle valve is provided to control the amount of intake air, a surge tank is provided downstream of the throttle valve. Thus, evaporative fuel (so-called, vapor) may be diluted by a large amount of air that is storable in the surge tank, so the need for an exclusive dilution device other than the surge tank is low. In contrast, in an independent throttle valve type in which a plurality of throttle valves are provided and actuated independently of one another, it is impossible to provide space for storing air, like a surge tank, for each cylinder. Thus, it is generally difficult to dilute evaporative fuel. Furthermore, when the absolute amount of intake air required by the internal combustion engine is small as in the case of idling, the amount of intake air that is usable for diluting evaporative fuel is also small. In addition, because the rotational speed of the internal combustion engine is controlled by the throttle valves during idling, it is necessary to ensure an allowable range within which the throttle valves are actuated to a closing side; however, in the independent throttle valve type, the leakage flow rate of the throttle valve for each cylinder needs to be considered and, therefore, the margin for closing the throttle valve necessarily decreases.

SUMMARY OF THE INVENTION

The invention provides an evaporative fuel treatment apparatus for an internal combustion engine, which is able to appropriately treat evaporative fuel while suppressing an abrupt change in air-fuel ratio.

A first aspect of the invention provides an evaporative fuel treatment apparatus for an internal combustion engine. The evaporative fuel treatment apparatus includes: a plurality of branch passages that respectively communicate with a plurality of cylinders; a plurality of throttle valves that are respectively provided in the plurality of branch passages, wherein each of the plurality of throttle valves is able to change the amount of air taken into a corresponding one of the plurality of cylinders; a balancing passage that communicates the plurality of branch passages with one another at positions downstream of the plurality of throttle valves; a communication portion that communicates the plurality of branch passages with one another at positions downstream of the plurality of throttle valves; a purge passage that introduces purge gas, containing evaporative fuel generated in a fuel tank, to the communication portion; an air supply passage that flows dilution air, which is used to dilute the evaporative fuel, into the purge passage; a first flow rate changing portion that is provided in the air supply passage and that is able to change the inflow of the dilution air; and a control unit that controls the first flow rate changing portion such that the inflow of the dilution air is changed on the basis of a comparison between the concentration of the evaporative fuel and a predetermined concentration determined on the basis of an operating state of the internal combustion engine.

According to the evaporative fuel treatment apparatus for an internal combustion engine in the first aspect, the balancing passage communicates the plurality of branch passages with one another at positions downstream of the plurality of throttle valves. The balancing passage implements the function of equalizing the pressures in the intake pipes corresponding to the respective cylinders, that is, a so-called balancing function. Specifically, the balancing passage is able to reduce variations in the amounts of air that flows through the throttle valves and the amounts of air taken into the respective cylinders. In other words, the negative pressures (that is, the amounts of intake air) in the respective cylinders downstream of the throttle valves are equalized. Note that the flow passage area according to the aspects of the invention means the cross-sectional area of a passage through which fluid flows.

The flow passage area of the communication portion may be smaller than the flow passage area of the balancing passage. With the above configuration, the pressure in the communication portion is higher than the pressure in each branch passage because of the flow resistance of the communication portion. Thus, steady flow is formed. Specifically, the communication portion may be a passage having a narrow diameter such that pulsation does not occur inside or considerably small pulsation occurs.

The purge passage utilizes the steady flow to introduce purge gas, containing evaporative fuel generated in the fuel tank, to each branch passage through the communication portion and finally introduces the purge gas to each cylinder. Thus, because purge gas is introduced into the branch passages, evaporative fuel may be treated without releasing the evaporative fuel to the atmosphere.

The plurality of throttle valves are respectively provided in the plurality of branch passages, and each of the plurality of throttle valves is able to change the amount of air taken into a corresponding one of the plurality of cylinders.

The air supply passage may flow dilution air, which is used to dilute the evaporative fuel, from an intake system upstream of the plurality of throttle valves into the purge passage. With this configuration, purge gas introduced into the branch passages is diluted with dilution air introduced through the air supply passage.

Particularly, under the control of the control unit, the first flow rate changing portion changes the flow rate of the dilution air on the basis of a comparison between the concentration of the evaporative fuel and a predetermined concentration determined on the basis of an operating state of the internal combustion engine. In other words, the flow rate of the dilution air is uniquely changed on the basis of a comparison in magnitude between the concentration of the evaporative fuel and the predetermined concentration. Here, the “operating state” according to the aspects of the invention may mean various states that quantitatively or qualitatively indicates the state of the internal combustion engine, such as the rotational speed of the internal combustion engine, the load on the internal combustion engine, such as the amount of fuel injection or the amount of intake air, the speed of a vehicle equipped with the internal combustion engine, or the driver's accelerator operation amount of a vehicle equipped with the internal combustion engine. In addition, the “concentration of evaporative fuel” according to the aspects of the invention may mean the concentration of evaporative fuel contained in purge gas or may basically mean “percent by weight” throughout. Alternatively, as long as the same evaporative fuel is referenced, the “concentration of evaporative fuel” may be percent by volume. Alternatively, in terms of the meaning that the relative rate of evaporative fuel in purge gas, the “concentration of evaporative fuel” may be dimensionless.

Specifically, when the concentration of evaporative fuel is higher than the above described predetermined concentration, the first flow rate changing portion increases the inflow of dilution air. On the other hand, when the concentration of evaporative fuel is lower than the above described predetermined concentration, the first flow rate changing portion reduces the inflow of dilution air. More specifically, the concentration of evaporative fuel may be controlled so as to converge on the predetermined concentration over time. Alternatively, the concentration of evaporative fuel may be controlled so as to be lower than the predetermined concentration. Thus, purge gas that contains a remarkably high concentration of evaporative fuel above the predetermined concentration is substantially or completely not introduced into the branch passages. Thus, it is possible to suppress variations in the air-fuel ratio due to purge gas among the branch passages and among the cylinders and, therefore, it is possible to appropriately prevent an abrupt change in the air-fuel ratio. Alternatively, purge gas that contains a remarkably low concentration of evaporative fuel below the predetermined concentration is substantially or completely not introduced into the branch passages. Thus, it is possible to appropriately prevent an abrupt change in the air-fuel ratio due to purge gas.

Particularly, the first flow rate changing portion increases the inflow of dilution air that flows through the air supply passage. Thus, it is possible to effectively prevent purge gas that contains an unnecessarily high concentration of evaporative fuel from being introduced to the branch passages due to a negative pressure generated at positions downstream of the throttle valves, for example, when the opening degrees of the throttle valves are reduced. Thus, the rotational speed of the internal combustion engine is easily maintained at a low rotational speed. Typically, in addition to the control of the opening degree of each throttle valve, the first flow rate changing portion adjusts the inflow of dilution air. Thus, it is possible to stabilize the control of the air-fuel ratio at a low rotational speed, such as during idling, and, therefore, it is possible to stabilize the operating state at a low rotational speed.

As described above, according to the evaporative fuel treatment apparatus for an internal combustion engine in the first aspect, because purge gas introduced into the branch passages is diluted by air introduced through the air supply passage, purge gas that contains a remarkably high concentration of evaporative fuel is substantially or completely not introduced into the branch passages. Thus, it is possible to suppress variations in the air-fuel ratio due to purge gas among the cylinders and, therefore, it is possible to appropriately prevent an abrupt change in the air-fuel ratio.

If, without changing the inflow of dilution air on the basis of the vapor concentration by the first flow rate changing portion, the opening degree of each throttle valve is controlled alone, high-concentration purge gas is drawn into the cylinders, so the air-fuel ratio varies among the cylinders due to the purge gas, and the air-fuel ratio abruptly changes. Alternatively, variations in the air-fuel ratio among the cylinders increase when the distribution of purge gas to the cylinders varies due to the influence of pulsation, or the like. This causes deterioration in drivability, such as engine vibration, or causes decrease in stability of the control of the air-fuel ratio. In addition, exhaust emission deteriorates.

A second aspect of the invention provides an evaporative fuel treatment apparatus for a multi-cylinder internal combustion engine that includes a pair of first and second banks respectively having a plurality of cylinders. The evaporative fuel treatment apparatus includes: a plurality of branch passages that respectively communicate with the plurality of cylinders in each bank; a plurality of throttle valves that are respectively provided in the plurality of branch passages in each bank, wherein each of the plurality of throttle valves is able to change the amount of air taken into a corresponding one of the plurality of cylinders; a balancing passage that communicates the plurality of branch passages with one another at positions downstream of the plurality of throttle valves in each bank; a communication portion that communicates the plurality of branch passages with one another at positions downstream of the plurality of throttle valves in the first bank; a purge passage that introduces purge gas, containing evaporative fuel generated in a fuel tank, to the communication portion in the first bank; an air supply passage that flows dilution air, which is used to dilute the evaporative fuel, into the purge passage in the first bank; a first flow rate changing portion that is provided in the air supply passage in the first bank and that is able to change the inflow of the dilution air; and a control unit that controls the opening degrees of the plurality of throttle valves in the second bank so that the amount of air taken into the second bank reduces by a differential flow rate, when an actual flow rate of dilution air that is changeable by the first flow rate changing portion is smaller than a predetermine flow rate of the dilution air used to reduce the concentration of the evaporative fuel below a predetermined concentration determined on the basis of an operating state of the internal combustion engine, wherein the differential flow rate is a difference between the predetermined flow rate and the actual flow rate.

According to the evaporative fuel treatment apparatus for an internal combustion engine in the second aspect, in a multi-cylinder internal combustion engine that includes a pair of first and second banks respectively having a plurality of cylinders, such as a V-engine, the plurality of branch passages respectively communicate with the plurality of cylinders in each bank. The plurality of throttle valves are respectively provided in the plurality of branch passages in each bank, and each of the plurality of throttle valves is able to change the amount of air taken into a corresponding one of the plurality of cylinders. The balancing passage communicates the plurality of branch passages with one another at positions downstream of the plurality of throttle valves in each bank. The communication portion in the first bank communicates the plurality of branch passages with one another at positions downstream of the plurality of throttle valves. The flow passage area (cross-sectional area) of the communication portion may be smaller than the flow passage area of the balancing passage. The purge passage in the first bank introduces purge gas, containing evaporative fuel generated in a fuel tank, to the communication portion. The air supply passage in the first bank flows dilution air, which is used to dilute the evaporative fuel, into the purge passage. The first flow rate changing portion in the first bank is provided in the air supply passage and is able to change the inflow of the dilution air. The dilution air may be air taken from an intake system upstream of the plurality of throttle valves.

Particularly, when an actual flow rate of dilution air that is changeable by the first flow rate changing portion is smaller than a predetermine flow rate of the dilution air used to reduce the concentration of the evaporative fuel below a predetermined concentration determined on the basis of an operating state of the internal combustion engine, under the control of the control unit, the opening degrees of the plurality of throttle valves in the second bank are controlled so that the amount of air taken into the second bank reduces by a differential flow rate, which is a difference between the predetermined flow rate and the actual flow rate. In addition, not only the differential in flow rate is controlled, but also a target number of revolutions of the engine is maintained.

If the inflow of dilution air used to dilute purge gas that contains evaporative fuel is insufficient, and when the flow rate of purge gas, containing evaporative fuel and flowing into the internal combustion engine, is restricted by the above described second flow rate changing portion, such as the purge gas variable throttle valve, in order to prevent engine stop and poor drivability due to imbalanced distribution of evaporative fuel among the cylinders, evaporative fuel contained in purge gas also reduces. As a result, the total amount of evaporative fuel drawn into the internal combustion engine reduces and, therefore, purge gas flows out to the atmosphere.

In contrast, according to the evaporative fuel treatment apparatus for an internal combustion engine in the second aspect, when an actual flow rate of dilution air that is changeable by the first flow rate changing portion is smaller than a predetermine flow rate of the dilution air used to reduce the concentration of the evaporative fuel below a predetermined concentration determined on the basis of an operating state of the internal combustion engine, under the control of the control unit, the opening degrees of the plurality of throttle valves in the second bank are controlled so that the amount of air taken into the second bank reduces by a differential flow rate, which is a difference between the predetermined flow rate and the actual flow rate. Thus, it is possible to additionally assign air, which is originally assigned to the second bank, to the first bank by the differential flow rate. Hence, in the first bank, it is possible to ensure dilution air by the above described predetermined flow rate. Thus, it is not necessary that the above second flow rate changing portion, such as a purge gas variable throttle valve, reduces the flow rate of purge gas introduced to the purge passage, and the total amount of evaporative fuel drawn into all the cylinders of the pair of banks is not reduced. Hence, it is possible to prevent purge gas from being released to the atmosphere.

Particularly, the necessary inflow of dilution air used to dilute evaporative fuel in the first bank is ensured while the total amount of air taken into both the banks is not varied. This makes it easy to maintain the rotational speed of the internal combustion engine at a low rotational speed and at a target rotational speed. This is because a drive shaft is shared between the first bank and the second bank, the total of the amount of air taken into the first bank and the amount of air taken into the second bank ensures the amount of air that achieves a target torque by which a low rotational speed may be maintained. Typically, in addition to the control of the opening degree of each throttle valve of the first bank and the adjustment to increase the inflow of dilution air by the first flow rate changing portion, by controlling each throttle valve of the second bank to reduce the opening degree, it is possible to stabilize the control of the air-fuel ratio at a low rotational speed, such as during idling, and, therefore, it is possible to stabilize the operating state at a low rotational speed.

A third aspect of the invention provides an evaporative fuel treatment apparatus for a multi-cylinder internal combustion engine that includes a pair of first and second banks respectively having a plurality of cylinders. The evaporative fuel treatment apparatus includes: a plurality of branch passages that respectively communicate with the plurality of cylinders in each bank; a plurality of throttle valves that are respectively provided in the plurality of branch passages in each bank, wherein each of the plurality of throttle valves is able to change the amount of air taken into a corresponding one of the plurality of cylinders; a balancing passage that communicates the plurality of branch passages with one another at positions downstream of the plurality of throttle valves in each bank; a communication portion that communicates the plurality of branch passages with one another at positions downstream of the plurality of throttle valves in each bank; a purge passage that introduces purge gas, containing evaporative fuel generated in a fuel tank, to the communication portion in the first bank; a first air supply passage that flows dilution air, which is used to dilute the evaporative fuel, from an intake system upstream of the plurality of throttle valves into the purge passage in the first bank; a second air supply passage that flows upstream air, in the second bank, from an intake system upstream of the plurality of throttle valves into the communication portion of the second bank; a first flow rate changing portion that is provided in the first air supply passage in the first bank and that is able to change the inflow of the dilution air; a second flow rate changing portion that is provided in the second air supply passage in the second bank and that is able to change the inflow of the upstream air; and a control unit that controls the first flow rate changing portion such that the inflow of the dilution air is changed on the basis of a comparison between the concentration of the evaporative fuel and a predetermined concentration determined on the basis of an operating state of the internal combustion engine, and that controls the second flow rate changing portion such that the inflow of the dilution air is equalized to the inflow of the upstream air.

According to the evaporative fuel treatment apparatus for an internal combustion engine in the third aspect, in a multi-cylinder internal combustion engine that includes a pair of first and second banks respectively having a plurality of cylinders, such as a V-engine, the plurality of branch passages respectively communicate with the plurality of cylinders in each bank.

The plurality of throttle valves are respectively provided in the plurality of branch passages in each bank, and each of the plurality of throttle valves is able to change the amount of air taken into a corresponding one of the plurality of cylinders. The balancing passage communicates the plurality of branch passages with one another at positions downstream of the plurality of throttle valves in each bank.

The communication portion communicates the plurality of branch passages with one another at positions downstream of the plurality of throttle valves in each bank. The flow passage area (cross-sectional area) of the communication portion may be smaller than the flow passage area of the balancing passage.

The purge passage introduces purge gas, containing evaporative fuel generated in a fuel tank, to the communication portion in the first bank.

The first air supply passage flows dilution air, which is used to dilute the evaporative fuel, from an intake system upstream of the plurality of throttle valves into the purge passage in the first bank.

The second air supply passage flows upstream air, in the second bank, from an intake system upstream of the plurality of throttle valves into the communication portion of the second bank.

The first flow rate changing portion is provided in the first air supply passage in the first bank and is able to change the inflow of the dilution air.

The second flow rate changing portion is provided in the second air supply passage in the second bank and is able to change the inflow of the upstream air.

Thus, the flow resistance of air that flows through the communication portion, the purge passage, the first air supply passage and the first flow rate changing portion in the first bank may be equalized to the flow resistance of air that flows through the communication portion, the second air supply passage and the second flow rate changing portion in the second bank. Thus, it is possible to coincide the amount of air that flows through the throttle valves and first air supply passage of the first bank with the amount of air that flows through the throttle valves and second air supply passage of the second bank.

Particularly, under the control of the control unit, the first flow rate changing portion changes the inflow of dilution air on the basis of a comparison between the concentration of the evaporative fuel and a predetermined concentration determined on the basis of an operating state of the internal combustion engine. In addition to this, under the control of the control unit, the second flow rate changing portion changes the inflow of upstream air so as to be equal to the inflow of dilution air.

If the flow resistance of air that flows through the communication portion, the purge passage, the first air supply passage and the first flow rate changing portion in the first bank cannot be made substantially equal to the flow resistance of air that flows through the communication portion, the second air supply passage and the second flow rate changing portion in the second bank, that is, if the amount of air that flows through the throttle valves and first air supply passage of the first bank does not coincide with the amount of air that flows through the throttle valves and second air supply passage of the second bank, the range of characteristics of the opening degree of each throttle valve in the first bank differs from the range of characteristics of the opening degree of each throttle valve in the second bank. Thus, for example, control of the air-fuel ratio at a low rotational speed, such as during idling, becomes unstable. In other words, if the first flow rate changing portion is provided for the first bank and no second flow rate changing portion is provided for the second bank, the operation of each throttle valve in the first bank, to which purge gas that contains evaporative fuel flows, causes relatively small rate of change in the amount of intake air against the opening degree of each throttle valve, and the amount of intake air does not change much only by varying the opening degree of each throttle valve in some degree. In contrast, the operation of each throttle valve in the second bank, to which no purge gas that contains evaporative fuel flows, causes relatively large rate of change in the amount of intake air in association with the opening degree of each throttle valve, and the amount of intake air relatively changes by a large amount even when the opening degree of each throttle valve is varied a little.

In detail, the resolution of control of the opening degree of each throttle valve is restricted by hardware. For example, 0.03 degree is the minimum unit of control of the opening degree of each throttle valve. Thus, when the throttle valves having largely different opening degree characteristics are used respectively in both banks, a variation in the amount of intake air per the above described minimum unit, that is, an increase or a decrease in actual rotational speed of the internal combustion engine, varies between both banks. Thus, there is a possibility that variations in rotational speed during idling may occur in at least one of the banks.

In contrast, according to the evaporative fuel treatment apparatus for an internal combustion engine in the third aspect, under the control of the control unit, the first flow rate changing portion changes the inflow of dilution air on the basis of a comparison between the concentration of the evaporative fuel and a predetermined concentration determined on the basis of an operating state of the internal combustion engine. In addition to this, under the control of the control unit, the second flow rate changing portion equalizes the inflow of dilution air to the inflow of upstream air. Thus, because it is possible to control both banks substantially in the same manner, it is possible to suppress variations in rotational speed due to a torque difference caused by a difference in the amount of air between both banks during idling.

BRIEF DESCRIPTION OF THE DRAWINGS

The features, advantages, and technical and industrial significance of this invention will be described in the following detailed description of example embodiments of the invention with reference to the accompanying drawings, in which like numerals denote like elements, and wherein:

FIG. 1 is a block diagram that shows a relevant portion of an internal combustion engine to which an evaporative fuel treatment apparatus according to a first embodiment of the invention is applied;

FIG. 2 is a schematic cross-sectional view that is taken along the line II-II in FIG. 1;

FIG. 3 is a flowchart that shows the control process flow of an ECU that controls the evaporative fuel treatment apparatus for the internal combustion engine according to the first embodiment of the invention;

FIG. 4A is a graph that schematically shows the relationship between an operating state of the internal combustion engine and a predetermined concentration according to the first embodiment of the invention;

FIG. 4B is a graph that schematically shows the relationship between the inflow of dilution air, which is determined on the basis of the predetermined concentration in FIG. 4A, and an operating state of the internal combustion engine;

FIG. 5A is a graph that shows changes in the operating state of the internal combustion engine according to the first embodiment of the invention;

FIG. 5B is a graph that shows changes in the inflow of dilution air according to the first embodiment of the invention;

FIG. 5C is a graph that shows changes in the concentration of evaporative fuel (so-called, vapor) according to the first embodiment of the invention;

FIG. 6 is a schematic diagram that shows the basic configuration of an evaporative fuel treatment apparatus for an internal combustion engine according to a second embodiment of the invention;

FIG. 7A is a bar graph that shows the distribution of the amount of intake air during normal idling and when dilution air is increased, in a pair of banks according to the second embodiment of the invention;

FIG. 7B is a graph that adds the differential flow rate to FIG. 7A;

FIG. 8 is a schematic diagram that shows the basic configuration of an evaporative fuel treatment apparatus for an internal combustion engine according to a third embodiment of the invention;

FIG. 9A is a bar graph that shows the distribution of the amount of intake air during idling in a pair of banks according to a comparative embodiment;

FIG. 9B is a graph that shows the relationship between the opening degree of a throttle valve and the amount of intake air through the throttle valve, in the pair of banks according to the comparative embodiment;

FIG. 9C is a bar graph that shows the distribution of the amount of intake air during idling in a pair of banks according to the third embodiment of the invention;

FIG. 10 is a flowchart that shows the operating principle of an evaporative fuel treatment apparatus for an internal combustion engine according to a fourth embodiment of the invention;

FIG. 11 is another flowchart that shows the operating principle of the evaporative fuel treatment apparatus for an internal combustion engine according to the fourth embodiment of the invention;

FIG. 12A is a graph that shows a common example of the relationship among a purge pressure, at a position downstream of a purge gas variable throttle valve and upstream of a purge passage, an intake pipe pressure, and the opening degree of a variable flow rate valve; and

FIG. 12B is a graph that shows a common example of the relationship between the flow rate of purge gas and a purge pressure at a position downstream of the purge gas variable throttle valve and upstream of the purge passage.

DETAILED DESCRIPTION OF EMBODIMENTS

First, the basic configuration of an evaporative fuel treatment apparatus for an internal combustion engine according to a first embodiment of the invention will be described with reference to FIG. 1 and FIG. 2. FIG. 1 is a block diagram that shows a relevant portion of the internal combustion engine to which the evaporative fuel treatment apparatus according to the first embodiment of the invention is applied. FIG. 2 is a schematic cross-sectional view that is taken along the line II-II in FIG. 1.

The internal combustion engine 1 includes cylinders 2, an intake passage 3, branch passages 3 a, a surge tank 3 b, throttle valves 7, a balancing passage 9, a purge passage 10, a communication portion 10 a, a mixing promoting portion 10 b, a canister 11, an air introducing pipe 11 a, an air supply passage 12, a purge gas variable throttle valve 14, a purge pressure sensor 15, an air-fuel ratio sensor 19, and a fuel tank 20.

The internal combustion engine 1 is an in-line four-cylinder spark ignition internal combustion engine (multi-cylinder internal combustion engine) in which the four cylinders 2 are arranged in one direction. Each cylinder 2 is connected to the intake passage 3 and an exhaust passage 4. The intake passage 3 includes the branch passages 3 a branched in correspondence with the cylinders 2 and the surge tank 3 b connected to the branch passages 3 a. An air cleaner 5 for filtering air is provided upstream of the surge tank 3 b. An air flow meter 6 is provided in the intake passage 3 downstream of the air cleaner 5 and upstream of a position at which the branch passages 3 a branch off. The air flow meter 6 is able to detect the flow rate of intake air. The throttle valve 7 is provided one by one in each branch passage 3 a. The opening degree of each throttle valve 7 may be adjusted so that the flow rate of intake air is adjustable. An injector 8 that injects fuel is provided downstream of each throttle valve 7 one by one in each branch passage 3 a. Each throttle valve 7 is attached to a valve shaft 7 a that extends through the branch passages 3 a, and the valve shaft 7 a is driven by an actuator 7 b for rotation. Thus, each throttle valve 7 serves as a so-called independent throttle valve. Note that the air flow meter 6 constitutes a specific example of an intake air flow rate detector according to the aspects of the invention.

As shown in FIG. 2, the balancing passage 9 is provided downstream of the throttle valves 7. The balancing passage 9 communicates the branch passages 3 a with one another at positions downstream of the throttle valves 7 in order to reduce a difference in pressure among the branch passages 3 a. In addition, the purge passage 10 is connected to the branch passages 3 a so as to face the balancing passage 9. The purge passage 10 introduces purge gas containing vapor (evaporative fuel) generated in the fuel tank 20. The purge passage 10 has the communication portion 10 a that communicates the branch passages 3 a with one another at positions downstream of the throttle valves 7. The flow passage area of the communication portion 10 a is smaller than the flow passage area of the balancing passage 9. The absolute size of the flow passage area of the communication portion 10 a may be set as appropriate. The flow passage area of the communication portion 10 a is desirably set to a size such that pulsation does not occur in the communication portion 10 a or considerably small pulsation occurs.

As shown in FIG. 1, the purge passage 10 is connected to the fuel tank 20 through the canister 11 that adsorbs evaporative fuel. The canister 11 is a known one that adsorbs evaporative fuel, introduced from the fuel tank 20, with internal activated carbon while air is introduced through the air introducing pipe 11 a that opens to the atmosphere.

The purge gas variable throttle valve 14 is provided downstream of the canister 11 toward the purge passage 10 as a purge gas flow rate changing portion that changes the flow rate of purge gas. The purge gas variable throttle valve 14 is able to adjust the opening degree so that the flow rate of purge gas introduced to the purge passage 10 is adjustable. Note that the purge gas variable throttle valve 14 is a specific example of a second flow rate changing portion according to the aspects of the invention.

The air supply passage 12 is connected between the purge gas variable throttle valve 14 and the purge passage 10, and introduces air into the purge passage 10. The other end of the air supply passage 12 is connected to the surge tank 3 b.

A variable flow rate valve 13 is provided in the air supply passage 12. The variable flow rate valve 13 serves as a flow rate changing portion that changes the flow rate of air flowing from the air supply passage 12 to the purge passage 10. The variable flow rate valve 13 is able to adjust the opening degree so that the flow rate of air flowing into the purge passage 10 is adjustable. As air flows from the air supply passage 12 into the purge passage 10, purge gas that flows through the purge passage 10 is diluted by the air. Note that the variable flow rate valve 13 is a specific example of a first flow rate changing portion according to the aspects of the invention.

The mixing promoting portion 10 b is provided downstream of a position at which the air supply passage 12 is connected to the purge passage 10. The mixing promoting portion 10 b promotes mixing of purge gas with air. The mixing promoting portion 10 b is formed of the meandering purge passage 10, and serves as a specific example of a mixing promoting portion according to the aspects of the invention.

The air-fuel ratio sensor 19 is provided in the exhaust passage 4. The air-fuel ratio sensor 19 may be configured to be able to estimate an air-fuel ratio in each cylinder 2.

The purge pressure sensor 15 measures a pressure at a position downstream of the purge gas variable throttle valve 14 and upstream of the purge passage 10. Note that the purge pressure sensor 15 is a specific example of a pressure measurement portion according to the aspects of the invention.

An engine control unit (ECU) 16 controls various components that constitute the internal combustion engine 1. Note that the ECU 16 is a specific example of a control unit according to the aspects of the invention.

According to the above embodiment, because the flow passage area of the communication portion 10 a is smaller than that of the balancing passage 9, the pressure in the communication portion 10 a is higher than that in each branch passage 3 a due to the flow resistance of the communication portion 10 a. Thus, steady flow is formed as shown by the arrows in FIG. 2. Purge gas is introduced into the branch passages 3 a using that steady flow, so evaporative fuel may be treated without being released to the atmosphere.

Particularly, according to the present embodiment, the flow rate of air used to dilute the purge gas that is introduced into the branch passages 3 a may be changed by the variable flow rate valve 13 provided in the air supply passage 12. Thus, when purge gas that contains a high concentration of evaporative fuel is introduced to the branch passages 3 a, the variable flow rate valve 13 increases the flow rate of air to thereby make it possible to suppress an abrupt change in the air-fuel ratio toward the rich side. Note that the air-fuel ratio is determined on the basis of the number of revolutions of the internal combustion engine and the load on the internal combustion engine.

On the other hand, when purge gas that contains a low concentration of evaporative fuel is introduced to the branch passages 3 a, the variable flow rate valve 13 reduces the flow rate of air to thereby make it possible to suppress an abrupt change in the air-fuel ratio toward the lean side. Specifically, the variable flow rate valve 13 provided in the air supply passage 12 is able to reduce the flow rate of air, flowing from the air supply passage 12 into the purge passage 10. This can prevent unnecessary air from flowing into the purge passage 10 when the opening degrees of the throttle valves 7 are reduced. Thus, the internal combustion engine 1 is easily maintained at a low rotational speed and, therefore, for example, the rotational speed of the internal combustion engine 1 is stable during idling.

In addition to or in place of this, as described above, the opening degree of the purge gas variable throttle valve 14 may be adjusted so that the flow rate of purge gas introduced to the purge passage 10 is adjustable. Then, when purge gas that contains an extremely high concentration of evaporative fuel is introduced to the branch passages 3 a, and when it is difficult to suppress an abrupt change in the air-fuel ratio toward the rich side only by increasing the flow rate of air using the variable flow rate valve 13, the purge gas variable throttle valve 14 reduces the flow rate of purge gas introduced to the purge passage 10. Thus, evaporative fuel introduced to the branch passages 3 a may be reduced, so it is possible to further appropriately and quickly suppress an abrupt change in the air-fuel ratio toward the rich side.

In addition, the position, at which air is taken out into the air supply passage 12, is located downstream of the air flow meter 6 and, therefore, purge gas is diluted by the air that is measured by the air flow meter 6. Thus, the amount of fuel injection may be determined by allowing for the amount of air used for dilution and, as a result, it is possible to further suppress variations in the air-fuel ratio. In addition, the air is taken out from the surge tank 3 b, so it is possible to prevent intake pulsation from influencing supply of air through the air supply passage 12. Furthermore, the mixing promoting portion 10 b is formed by meandering the purge passage 10. Thus, no additional component other than the purge passage 10 is required, so the number of components that implement the mixing promoting portion may be reduced.

Next, the operation principle of the evaporative fuel treatment apparatus for an internal combustion engine according to the first embodiment of the invention will be described with reference to FIG. 3 to FIG. 5C. FIG. 3 is a flowchart that shows the control process flow of the ECU that controls the evaporative fuel treatment apparatus for an internal combustion engine according to the first embodiment of the invention. FIG. 4A is a graph that schematically shows the relationship between an operating state of the internal combustion engine and a predetermined concentration according to the first embodiment of the invention. FIG. 4B is a graph that schematically shows the relationship between the inflow of dilution air, which is determined on the basis of the predetermined concentration in FIG. 4A, and an operating state of the internal combustion engine. FIG. 5A is a graph that shows changes in the operating state of the internal combustion engine according to the first embodiment of the invention. FIG. 5B is a graph that shows changes in the inflow of dilution air according to the first embodiment of the invention. FIG. 5C is a graph that shows changes in the concentration of evaporative fuel (so-called, vapor) according to the first embodiment of the invention. Note that the ordinate axis of FIG. 5A represents a vehicle speed (km/h), and the abscissa axis of FIG. 5A represents time. The ordinate axis of FIG. 5B represents the inflow of dilution air, and the abscissa axis of FIG. 5B represents time. The ordinate axis of FIG. 5C represents a vapor concentration (%), and the abscissa axis of FIG. 5C represents time.

As shown in FIG. 3, under the control of the ECU 16, it is determined whether an execution condition for executing purge control in which purge gas containing evaporative fuel is appropriately diluted by dilution air is satisfied (step S101). When it is determined that the execution condition for executing the purge control is satisfied (step S101: Yes), under the control of the ECU 16, the flow rate of purge gas introduced to the purge passage 10 is determined on the basis of the operating state of the internal combustion engine, and then the opening degree of the purge gas variable throttle valve 14 and the duration during which the opening degree of the purge gas variable throttle valve 14 is maintained, so-called purge rate pgr, are calculated (step S102).

Subsequently, under the control of the ECU 16, the concentration of vapor (vapor concentration) fgpg per one percent of the purge rate is calculated (step S103). Note that at this time, the average fgpgsm of the vapor concentration fgpg (average vapor concentration fgpgsm) for a predetermined duration may be calculated. Particularly, the vapor concentration fgpg or the average vapor concentration fgpgsm is a specific example of a predetermined concentration determined on the basis of an operation state of the internal combustion engine according to the aspects of the invention. Specifically, as shown in FIG. 4A, the predetermined concentration may be determined on the basis of the number of revolutions or rotational speed of the internal combustion engine and the load on the internal combustion engine, such as the amount of fuel injection or the amount of intake air. FIG. 4A shows the relationship among the number of revolutions of the internal combustion engine, the load on the internal combustion engine, and four predetermined concentrations A, B, C and D.

Then, under the control of the ECU 16, the amount of required dilution air qiscvt (required dilution air amount qiscvt) is calculated on the basis of the average vapor concentration fgpgsm (step S104). Note that at this time, the average qiscvtsm of the required dilution air amount qiscvt (average required dilution air amount qiscvtsm) for a predetermined duration may be calculated. Specifically, as shown in FIG. 4B, the required dilution air amount qiscvt or the average required dilution air amount qiscvtsm may be determined on the basis of the number of revolutions or rotational speed of the internal combustion engine and the load on the internal combustion engine, such as the amount of fuel injection or the amount of intake air, which are variables used to determine the vapor concentration. FIG. 4B shows the relationship among the number of revolutions of the internal combustion engine, the load on the internal combustion engine, and four required dilution air amounts (dilution air amounts) A′, B′, C′ and D′.

Next, under the control of the ECU 16, it is determined whether an actual vapor concentration is higher than a predetermined concentration (step S105). When it is determined that the actual vapor concentration is higher than the predetermined concentration (step S105: Yes), the required dilution air amount calculated as described above is multiplied by an increasing correction coefficient (step S106). Typically, the increasing correction coefficient may be larger than 1, for example, “1.5”.

On the other hand, as a result of the determination in step S105, when it is determined that the actual vapor concentration is not larger than the predetermined concentration (step S105: No), the required dilution air amount calculated as described above is multiplied by a reducing correction coefficient (step S107). Typically, the reducing correction coefficient may be smaller than 1, for example, “0.8”.

Note that the above described operating principle may be a feedback control or may be a feedforward control as long as the inflow of dilution air is changed on the basis of whether the vapor concentration is high or low.

As described above, according to the first embodiment, because purge gas introduced into the branch passages is diluted by air introduced through the air supply passage, purge gas that contains a remarkably high concentration of evaporative fuel is substantially or completely not introduced into the branch passages. Thus, it is possible to suppress variations in the air-fuel ratio due to purge gas among the cylinders and, therefore, it is possible to appropriately prevent an abrupt change in the air-fuel ratio.

Specifically, as shown in FIG. 5A, in a state where a vehicle equipped with the internal combustion engine according to the first embodiment changes in speed, when the vapor concentration remarkably increases, as shown in FIG. 5B, the variable flow rate valve 13 appropriately increases the inflow of dilution air on the basis of the increased vapor concentration. Thus, as shown in FIG. 5C, it is possible to remarkably suppress variations in the vapor concentration.

If, without changing the inflow of dilution air on the basis of the vapor concentration by the variable flow rate valve 13, the opening degree of each throttle valve is controlled alone, as shown by the dotted line corresponding to the comparative embodiment in FIG. 5C, high-concentration purge gas is drawn into the cylinders, so the air-fuel ratio varies among the cylinders due to the purge gas, and the air-fuel ratio abruptly changes. Alternatively, in the comparative embodiment, variations in the air-fuel ratio among the cylinders increase when the distribution of purge gas to the cylinders varies due to the influence of pulsation, or the like. This causes deterioration in drivability, such as engine vibration, or causes decrease in stability in the control of the air-fuel ratio. In addition, exhaust emission deteriorates.

In contrast, in the first embodiment, in addition to the control of the opening degree of each throttle valve, the variable flow rate valve 13 changes the inflow of dilution air on the basis of the concentration of evaporative fuel (vapor concentration). Thus, the influence of variations in the concentration of purge gas is reduced, and control of the air-fuel ratio in an operating state in which the amount of intake air is small, such as during idling, is stabilized. Hence, it is possible to stabilize the operating state at a low rotational speed.

Next, an evaporative fuel treatment apparatus for an internal combustion engine according to a second embodiment of the invention will be described with reference to FIG. 6 to FIG. 7B. FIG. 6 is a schematic diagram that shows the basic configuration of the evaporative fuel treatment apparatus for an internal combustion engine according to the second embodiment of the invention. FIG. 7A and FIG. 7B are bar graphs that show the distribution of the amount of intake air during normal idling and when dilution air is increased, in a pair of banks according to the second embodiment of the invention. Note that in the second embodiment, like reference numerals denote like components to those of the above described first embodiment, and the description thereof will be omitted where appropriate.

As shown in FIG. 6, a right bank bk1 that constitutes the internal combustion engine 1 includes cylinders (not shown), an intake passage 3, branch passages 3 a, a surge tank 3 b, throttle valves (not shown), a balancing passage 9, a purge passage 10, a communication portion 10 a, a mixing promoting portion 10 b, a canister 11, an air introducing pipe 11 a, an air supply passage 12, a variable flow rate valve 13, a purge gas variable throttle valve 14, a purge pressure sensor (not shown), an air-fuel ratio sensor (not shown), and a fuel tank 20. Note that the right bank bk1 is a specific example of a “first bank” according to the aspects of the invention.

In addition, a left bank bk2 that constitutes the internal combustion engine 1 includes cylinders (not shown), an intake passage 3, branch passages 3 a, a surge tank 3 b, throttle valves (not shown), a balancing passage 9, an air-fuel ratio sensor (not shown), and a fuel tank (not shown). Note that the left bank bk2 is a specific example of a “second bank” according to the aspects of the invention.

Particularly, in the second embodiment, when the actual flow rate of dilution air that is changeable by the variable flow rate valve 13 is smaller than the above described required dilution air amount used to decrease the concentration of evaporative fuel (vapor concentration) below a predetermined concentration that is determined on the basis of the operating state of the internal combustion engine, under the control of the ECU 16, the opening degrees of the plurality of throttle valves in the left bank bk2 are controlled so that the amount of air taken into the left bank bk2 is reduced by a differential flow rate, which is a difference between the required amount and the actual flow rate. In addition, not only the differential in flow rate is controlled, but also a target number of revolutions of the engine is maintained. Note that the required dilution air amount is a specific example of a “predetermined flow rate of dilution air” according to the aspects of the invention.

If the inflow of dilution air used to dilute purge gas that contains evaporative fuel (vapor) is insufficient, and when the flow rate of purge gas, containing evaporative fuel and flowing into the internal combustion engine, is restricted by the above described second flow rate changing portion, such as the purge gas variable throttle valve, in order to prevent engine stop and poor drivability due to imbalanced distribution of evaporative fuel among the cylinders, evaporative fuel contained in purge gas also reduces. As a result, the total amount of evaporative fuel drawn into the internal combustion engine reduces and, therefore, purge gas may flow out to the atmosphere.

In contrast, according to the second embodiment, when the actual flow rate of dilution air that is changeable by the variable flow rate valve 13 is smaller than the above described required dilution air amount used to decrease the concentration of evaporative fuel below a predetermined concentration that is determined on the basis of the operating state of the internal combustion engine, under the control of the ECU 16, the opening degrees of the plurality of throttle valves in the left bank bk2 are controlled so that the amount of air taken into the left bank bk2 is reduced by a differential flow rate, which is a difference between the required amount and the actual flow rate. Thus, it is possible to additionally assign air, which is originally assigned to the left bank bk2, to the right bank bk1 by the differential flow rate. Hence, in the right bank bk1, it is possible to ensure the above described required dilution air amount. Thus, the purge gas variable throttle valve 14 need not to reduce the flow rate of purge gas introduced to the purge passage, and the total amount of evaporative fuel drawn into all the cylinders of the pair of banks is not reduced. Hence, it is possible to reliably prevent purge gas from being released to the atmosphere.

Specifically, as shown in FIG. 7A, during normal idling, the amount of air taken into the right bank is substantially equal to the amount of air taken into the left bank. At this time, the amount of air taken into the right bank is the sum of the following four different intake air amounts. That is, the amount of air taken into the right bank is the sum of the amount of intake air from the variable flow rate valve, the amount of intake air from the throttle valves, the flow rate of air that leaks from the throttle valves and the flow rate of air that leaks from the variable flow rate valve. On the other hand, the amount of air taken into the left bank is the sum of the following two different intake air amounts. That is, the amount of air taken into the left bank is the sum of the amount of intake air from the throttle valves and the flow rate of air that leaks from the throttle valves.

Particularly, when the inflow of dilution air to the purge passage is increased in the right bank, the amount of air taken into the right bank is larger by the differential flow rate corresponding to the above increase than the amount of air taken into the left bank. At this time, in the right bank, among the above described four different intake air amounts, the amount of intake air from the variable flow rate valve increases by the differential flow rate. On the other hand, in the left bank, between the above described two different intake air amounts, the amount of intake air from the throttle valves reduces by the differential flow rate.

Particularly, the necessary inflow of dilution air used to dilute evaporative fuel in the right bank bk1 is ensured while the total amount of air taken into both banks is not varied. This makes it easy to maintain the rotational speed of the internal combustion engine at a low rotational speed and at a target rotational speed. This is because a drive shaft is shared between the right bank bk1 and the left bank bk2, the total of the amount of air taken into the right bank bk1 and the amount of air taken into the left bank bk2 ensures the amount of air that achieves a target torque by which a low rotational speed may be maintained. Typically, in addition to the control of the opening degree of each throttle valve of the right bank bk1 and the adjustment to increase the inflow of dilution air by the variable flow rate valve 13, by controlling each throttle valve of the left bank bk2 to reduce the opening degree, it is possible to stabilize the control of the air-fuel ratio at a low rotational speed, such as during idling, and, therefore, it is possible to stabilize the operating state at a low rotational speed.

Note that under the control of the ECU, the opening degrees of the plurality of throttle valves in the left bank are controlled on the basis of the following conditions. That is, the amount of air taken into the left bank is determined so that combustion in the cylinders of the left bank is occurable and/or the amount of air taken into the left bank is larger than a lower limit intake air amount determined on the basis of the air-fuel ratio required in the left bank. Thus, it is possible to avoid the operating state in which misfire occurs in the left bank or a torque assigned to the left bank cannot be generated because of a reduction in the amount of intake air in the left bank. Thus, it is possible to generate a target torque by the combination of both banks of the internal combustion engine.

Next, an evaporative fuel treatment apparatus for an internal combustion engine according to a third embodiment of the invention will be described with reference to FIG. 8 to FIG. 9C. FIG. 8 is a schematic diagram that shows the basic configuration of the evaporative fuel treatment apparatus for an internal combustion engine according to the third embodiment of the invention. FIG. 9A is a bar graph that shows the distribution of the amount of intake air during idling in a pair of banks according to a comparative embodiment. FIG. 9B is a graph that shows the relationship between the opening degree of a throttle valve and the amount of intake air through the throttle valve, in the pair of banks according to the comparative embodiment. FIG. 9C is a bar graph that shows the distribution of the amount of intake air during idling in a pair of banks according to the third embodiment of the invention. Note that in the third embodiment, like reference numerals denote like components to those of the above described first and second embodiments, and the description thereof will be omitted where appropriate.

As shown in FIG. 8, a right bank bk1 that constitutes the internal combustion engine 1 includes cylinders (not shown), an intake passage 3, branch passages 3 a, a surge tank 3 b, throttle valves (not shown), a balancing passage 9, a purge passage 10, a communication portion 10 a, a mixing promoting portion 10 b, a canister 11, an air introducing pipe 11 a, an air supply passage 12, a variable flow rate valve 13, a purge gas variable throttle valve 14, a purge pressure sensor (not shown), an air-fuel ratio sensor (not shown), and a fuel tank 20. Note that the right bank bk1 is another specific example of a “first bank” according to the aspects of the invention.

In addition, a left bank bk2 that constitutes the internal combustion engine 1 includes cylinders (not shown), an intake passage 3, branch passages 3 a, a surge tank 3 b, throttle valves (not shown), a balancing passage 9, a purge passage 10, a communication portion 10 a, a mixing promoting portion 10 b, an air supply passage 12, a variable flow rate valve 13, an air-fuel ratio sensor (not shown), and a fuel tank (not shown). Note that the left bank bk2 is another specific example of a “second bank” according to the aspects of the invention.

Thus, the flow resistance of air that flows through the communication portion 10 a, the purge passage 10, the air supply passage 12 and the variable flow rate valve 13 in the right bank bk1 may be equalized to the flow resistance of air that flows through the communication portion 10 a, the air supply passage 12 and the variable flow rate valve 13 in the left bank bk2. Thus, it is possible to coincide the amount of air that flows through the throttle valves and the air supply passage 12 in the right bank bk1 with the amount of air that flows through the throttle valves and the air supply passage 12 in the left bank bk2. Particularly, it is desirable because the flow resistances of air is equalized by providing the mixing promoting portion for each of the right bank bk1 and the left bank bk2.

Particularly, under the control of the ECU 16, the variable flow rate valve 13 of the right bank bk1 changes the inflow of dilution air on the basis of a comparison between the concentration of evaporative fuel and a predetermined concentration determined on the basis of the operating state of the internal combustion engine. In addition to this, under the control of the ECU 16, the variable flow rate valve 13 of the left bank bk2 changes the inflow of upstream air so as to be equal to the inflow of the dilution air.

If the flow resistance of air that flows through the communication portion 10 a, the purge passage 10, the air supply passage 12 and the variable flow rate valve 13 in the right bank bk1 cannot be made substantially equal to the flow resistance of air that flows through the communication portion 10 a, the purge passage 10, the air supply passage 12 and the variable flow rate valve 13 in the left bank bk2, that is, if the amount of air that flows through the throttle valves and the air supply passage 12 in the right bank does not coincide with the amount of air that flows through the throttle valves and the air supply passage 12 in the left bank, the range of characteristics of the opening degree of each throttle valve in the right bank differs from the range of characteristics of the opening degree of each throttle valve in the left bank. Thus, for example, control of the air-fuel ratio at a low rotational speed, such as during idling, becomes unstable. In other words, if the variable flow rate valve 13 is provided for the right bank and no variable flow rate valve 13 is provided for the left bank, as shown by the black solid curve S1 in FIG. 9B, the operation of each throttle valve in the right bank, to which purge gas containing evaporative fuel flows, causes relatively small rate of change in the amount of intake air against the opening degree of each throttle valve, and the amount of intake air does not change much only by varying the opening degree of each throttle valve in some degree. On the other hand, as shown by the black solid curve S2 in FIG. 9B, the operation of each throttle valve in the left bank, to which no purge gas containing evaporative fuel flows, causes relatively large rate of change in the amount of intake air against the opening degree of each throttle valve, and the amount of intake air changes by a large amount even when the opening degree of each throttle valve is varied a little. Specifically, as shown in FIG. 9A, when the amount of air taken into the right bank is substantially equalized to the amount of air taken into the left bank during idling, the amount of air taken into the right bank is the sum of four different intake air amounts (that is, the amount of intake air from the variable flow rate valve, the amount of intake air from the throttle valves, the flow rate of air that leaks from the throttle valves, and the flow rate of air that leaks from the variable flow rate valve). On the other hand, the amount of air taken into the left bank is the sum of two different intake air amounts (that is, the amount of intake air from the throttle valves and the flow rate of air that leaks from the throttle valves). Thus, because in the right bank, the percentage of the amount of intake air from the throttle valves and the flow rate of air that leaks from the throttle valves is small, the control of the opening degree of the throttle valves of the right bank less influences the amount of intake air of the right bank than that of the left bank. On the other hand, because in the left bank, the percentage of the amount of intake air from the throttle valves and the flow rate of air that leaks from the throttle valves is substantially 100%, the control of the opening degree of the throttle valves of the right bank remarkably more influences the amount of intake air of the right bank than that of the right bank.

In detail, the resolution of control of the opening degree of each throttle valve is restricted by hardware. For example, 0.03 degree is the minimum unit of control of the opening degree of each throttle valve. Thus, when the throttle valves having largely different opening degree characteristics are used respectively in both banks, a variation in the amount of intake air per the above described minimum unit, that is, an increase or a decrease in actual rotational speed of the internal combustion engine varies between both banks. Thus, there is a possibility that variations in rotational speed during idling may occur in at least one of the banks.

In contrast, according to the third embodiment, under the control of the ECU 16, the variable flow rate valve 13 in the right bank changes the inflow of dilution air on the basis of a comparison between the concentration of evaporative fuel and a predetermined concentration determined on the basis of the operating state of the internal combustion engine. In addition to this, under the control of the ECU 16, the variable flow rate valve 13 in the left bank changes the inflow of upstream air so as to be equalized to the inflow of dilution air. Thus, because both banks may be controlled substantially in the same manner, it is possible to suppress variations in rotational speed due to a torque difference caused by a difference in the amount of intake air between both banks during idling. Specifically, because the same range of the opening degree characteristics of each throttle valve may be used in both banks of the internal combustion engine, it is possible to stabilize the control of the air-fuel ratio at a low rotational speed, such as during idling, in both banks.

Specifically, as shown in FIG. 9C, the amount of air taken into the right bank is the sum of four different intake air amounts (that is, the amount of intake air from the variable flow rate valve, the amount of intake air from the throttle valves, the flow rate of air that leaks from the throttle valves, and the flow rate of air that leaks from the variable flow rate valve). Almost similarly, the amount of air taken into the left bank is the sum of four different intake air amounts (that is, the amount of intake air from the variable flow rate valve, the amount of intake air from the throttle valves, the flow rate of air that leaks from the throttle valves, and the flow rate of air that leaks from the variable flow rate valve). In this way, the percentage of the amount of intake air from the throttle valves and the flow rate of air that leaks from the throttle valves is substantially equalized between the right bank and the left bank. Thus, under the simple control of the ECU 16, the control of the opening degrees of the throttle valves of the right bank is substantially similar to the control of the opening degrees of the throttle valves of the left bank while, for example, during idling, it is possible to substantially equalize the amount of air taken into the right bank with the amount of air taken in to the left bank.

Next, an evaporative fuel treatment apparatus for an internal combustion engine according to a fourth embodiment of the invention will be described with reference to FIG. 10 to FIG. 12B. FIG. 10 is a flowchart that shows the operating principle of the evaporative fuel treatment apparatus for an internal combustion engine according to the fourth embodiment of the invention. FIG. 11 is another flowchart that shows the operating principle of the evaporative fuel treatment apparatus for an internal combustion engine according to the fourth embodiment of the invention. Note that in the fourth embodiment, like reference numerals denote like components to those of the above described first to third embodiments, and the description thereof will be omitted where appropriate. FIG. 12A is a graph that shows a common example of the relationship among a purge pressure, at a position downstream of a purge gas variable throttle valve 14 and upstream of a purge passage 10, an intake pipe pressure, and the opening degree of a variable flow rate valve. FIG. 12B is a graph that shows a common example of the relationship between the flow rate of purge gas and a purge pressure at a position downstream of the purge gas variable throttle valve 14 and upstream of the purge passage 10.

As shown in FIG. 10, under the control of the ECU 16, it is determined whether an execution condition for executing purge control in which purge gas containing evaporative fuel is appropriately diluted by dilution air is satisfied (step S201). When it is determined that the execution condition for executing the purge control is satisfied (step S201: Yes), under the control of the ECU 16, it is determined whether an abnormality is occurring in the purge pressure sensor 15 (step S202). When an abnormality is occurring in the purge pressure sensor 15 (step S202: Yes), under the control of the ECU 16, a purge pressure t_mv at a position downstream of the purge gas variable throttle valve 14 and upstream of the purge passage 10 is estimated on the basis of the operating state of the internal combustion engine, such as the rotational speed of the internal combustion engine and the opening degree of each throttle valve of the internal combustion engine. Particularly, at the time of the estimation, the opening degree of the variable flow rate valve 13 is set to a fixed value to thereby control the inflow of dilution air to a constant value. This can compensate for general characteristics that the correlation greatly changes between the purge pressure and the intake pipe pressure at a position downstream of the purge gas variable throttle valve 14 in accordance with a variation in the opening degree of the variable flow rate valve 13, as shown by the opening degree a, opening degree b, opening degree c, opening degree d and opening degree e in FIG. 12A. Note that the control process of the variable flow rate valve 13 will be described later. Thus, under the condition that the inflow of dilution air is a constant value, the ECU 16 is able to highly accurately estimate a purge pressure t_mv at a position downstream of the purge gas variable throttle valve 14 and upstream of the purge passage 10. Thus, because the flow rate of evaporative fuel may be highly accurately determined on the basis of the estimated purge pressure, by controlling the flow rate of purge gas on the basis of the inflow of dilution air fixed as described above (fixed to a constant value), it is possible to prevent purge gas containing a remarkably high concentration of evaporative fuel from being introduced into each branch passage. Hence, it is possible to suppress variations in the air-fuel ratio due to purge gas among the cylinders and, as a result, it is possible to appropriately prevent an abrupt change in the air-fuel ratio. This is because, as shown in FIG. 12B, the flow rate of purge gas may be highly accurately predicted on the basis of the highly accurately estimated purge pressure.

On the other hand, as a result of the determination of the above described step S202, when no abnormality is occurring in the purge pressure sensor 15 (step S202: No), because the purge pressure sensor 15 operates normally, the atmospheric pressure is added to the measured value of the purge pressure sensor 15, and the standard atmospheric pressure is subtracted from the resultant value. Thus, the ECU 16 corrects a purge pressure t_mv at a position downstream of the purge gas variable throttle valve 14 and upstream of the purge passage 10 (step S204).

Specifically, the correction value (ppg) of the purge pressure t_mv is derived from the following Expression (1).

Correction value of purge pressure t _(—) mv=klsm/{klmaxb+(1−kpa)}  (1)

-   where klsm denotes an intake air amount, -   klmaxb denotes the amount of air when the throttle valve is fully     opened, and -   kpa denotes an atmospheric pressure correction coefficient.

Next, under the control of the ECU 16, the opening degree of the purge gas variable throttle valve 14 and the purge rate, which is a duration during which the opening degree is maintained, are determined on the basis of the estimated or corrected purge pressure t_mv and the number of revolutions of the internal combustion engine. At this time, the relationship among the estimated or corrected purge pressure t_mv, the number of revolutions of the internal combustion engine, and the purge rate may be uniquely determined through a map, function, simulation, or the like. Note that the purge rate may be replaced with a purge rate that is obtained by dividing the flow rate of purge gas when the duty ratio of the purge gas variable throttle valve 14 is 100% by the intake air amount, that is, a so-called full open purge rate.

Next, the control process of the variable flow rate valve 13 will be described with reference to FIG. 11.

As shown in FIG. 11, under the control of the ECU 16, the controlled variable of the opening degree of the variable flow rate valve 13 during idling is calculated (step S301).

Subsequently, under the control of the ECU 16, it is determined whether an execution condition for executing purge control in which purge gas containing evaporative fuel is appropriately diluted by dilution air is satisfied (step S302). When it is determined that the execution condition for executing the purge control is satisfied (step S302: Yes), under the control of the ECU 16, it is determined whether an abnormality is occurring in the purge pressure sensor 15 (step S303). When an abnormality is occurring in the purge pressure sensor 15 (step S303: Yes), under the control of the ECU 16, the opening degree of the variable flow rate valve 13 is set to a fixed value to thereby control the inflow of dilution air to a constant value. Alternatively, under the control of the ECU 16, the opening degree of the variable flow rate valve 13 is restricted to thereby limit the inflow of dilution air (step S304).

Then, under the control of the ECU 16, the controlled variable of the opening degree of each throttle valve during idling is calculated and determined (step S305).

After that, under the control of the ECU 16, the opening degree of the variable flow rate valve 13 is calculated and determined on the basis of the above described controlled variable of the opening degree of each throttle valve (step S306).

Here, the purge pressure sensor 15 that measures the pressure at a position downstream of the purge gas variable throttle valve 14 and upstream of the purge passage 10 will be described in detail. According to the research by the inventors of the application, evaporative fuel is drawn into the internal combustion engine by the negative pressure in the intake pipe, so the amount of purge gas drawn into the cylinder increases as the amount of air that flows through the corresponding throttle valve reduces. In this case, in regard to the air-fuel ratio in each cylinder, the cylinder draws a larger amount of evaporative fuel when the amount of air that flows through the corresponding throttle valve is small despite a rich air-fuel ratio. This promotes the air-fuel ratio to be further rich. Then, to solve this problem, the communication portion is provided separately from the balancing passage, and the diameter of the communication portion (for example, 4 mm diameter) is reduced. Thus, the balancing function is suppressed to improve equal distribution. In this case, the communication portion serves as an orifice (air-flow resistance), and the pressure in the intake pipe is lower than the pressure in the purge passage 10 and is lower than the pressure downstream of the purge gas variable throttle valve 14. In addition, to further increase the purge rate, or to increase the pressure in the purge passage, fresh air is drawn into the purge passage through the variable flow rate valve 13 and mixed with purge gas. In this case, a variation in the amount of fresh air introduced through the variable flow rate valve 13 influences the pressure in the purge passage 10 and the pressure downstream of the purge gas variable throttle valve 14. In addition, as for the characteristics of the independent throttle valve, the average of the pressure in the intake pipe is determined on the basis of a negative pressure generated at the time when an intake valve is opened and a pressure difference based on the opening degree of the throttle valve when the intake valve is closed thereafter. Generally, there is no linear correlation between the amount of intake air and the average pressure in the intake pipe. On the basis of the above principle, there is no correlation between the pressure downstream of the purge gas variable throttle valve 14, which is a variable parameter of the flow rate of purge gas, and the amount of intake air. For this reason, it is technically difficult to estimate the pressure downstream of the purge gas variable throttle valve 14 on the basis of the amount of intake air and the number of revolutions or rotational speed of the internal combustion engine. In contrast, according to the fourth embodiment, the purge pressure sensor 15 is provided and directly measures the pressure at a position downstream of the purge gas variable throttle valve 14 and upstream of the purge passage 10. Thus, it is possible to accurately control the variation of the amount of drawn purge gas.

The aspects of the invention are not limited to the above described embodiments but they may be modified into various alternative embodiments. For example, the aspects of the invention are not limited to a gasoline engine. The aspects of the invention may be applied to various internal combustion engines that utilize diesel or other fuel instead.

The aspects of the invention are not limited to the embodiments described above. The aspects of the invention may be appropriately modified without departing from the scope or spirit of the invention interpreted from the appended claims and entire specification. The scope of the invention also encompasses the thus modified evaporative fuel treatment apparatus for an internal combustion engine.

While the invention has been described with reference to example embodiments thereof, it should be understood that the invention is not limited to the example embodiments or constructions. To the contrary, the invention is intended to cover various modifications and equivalent arrangements. In addition, while the various elements of the example embodiments are shown in various combinations and configurations, which are example, other combinations and configurations, including more, less or only a single element, are also within the spirit and scope of the invention. 

1. An evaporative fuel treatment apparatus for an internal combustion engine, comprising: a plurality of branch passages that respectively communicate with a plurality of cylinders; a plurality of throttle valves that are respectively provided in the plurality of branch passages, wherein each of the plurality of throttle valves is able to change the amount of air taken into a corresponding one of the plurality of cylinders; a balancing passage that communicates the plurality of branch passages with one another at positions downstream of the plurality of throttle valves; a communication portion that communicates the plurality of branch passages with one another at positions downstream of the plurality of throttle valves; a purge passage that introduces purge gas, containing evaporative fuel generated in a fuel tank, to the communication portion; an air supply passage that flows dilution air, which is used to dilute the evaporative fuel, into the purge passage; a first flow rate changing portion that is provided in the air supply passage and that is able to change the inflow of the dilution air; and a control unit that controls the first flow rate changing portion such that the inflow of the dilution air is changed on the basis of a comparison between the concentration of the evaporative fuel and a predetermined concentration determined on the basis of an operating state of the internal combustion engine.
 2. The evaporative fuel treatment apparatus for an internal combustion engine according to claim 1, wherein the flow passage area of the communication portion is smaller than the flow passage area of the balancing passage.
 3. The evaporative fuel treatment apparatus for an internal combustion engine according to claim 1, wherein the dilution air is air taken from an intake system upstream of the plurality of throttle valves.
 4. The evaporative fuel treatment apparatus for an internal combustion engine according to claim 1, further comprising a second flow rate changing portion that changes the flow rate of the purge gas introduced from the fuel tank to the purge passage, wherein the control unit controls the second flow rate changing portion such that the flow rate of the purge gas introduced to the purge passage reduces, when an actual flow rate of dilution air, which is changeable by the first flow rate changing portion, is smaller than a predetermined flow rate of the dilution air by which the concentration of the evaporative fuel is reduced below the predetermined concentration.
 5. The evaporative fuel treatment apparatus for an internal combustion engine according to claim 1, further comprising a concentration determination portion that determines the concentration of the evaporative fuel, wherein the control unit controls the first flow rate changing portion such that the inflow of the dilution air increases, when the determined concentration of the evaporative fuel is higher than the predetermined concentration, and controls the first flow rate changing portion such that the inflow of the dilution air reduces, when the determined concentration of the evaporative fuel is lower than the predetermined concentration.
 6. The evaporative fuel treatment apparatus for an internal combustion engine according to claim 1, further comprising an intake air flow rate detector that is provided in the intake system upstream of a branch position at which the branch passages branch off and that detects the flow rate of intake air, wherein the air supply passage takes air out of the intake system downstream of the intake air flow rate detector and upstream of the branch position and flows the taken out air into the purge passage.
 7. The evaporative fuel treatment apparatus for an internal combustion engine according to claim 6, further comprising a surge tank that is arranged downstream of the intake air flow rate detector and upstream of the branch position and that forms part of the intake system, wherein the air supply passage takes air out of the surge tank and flows the taken out air into the purge passage.
 8. The evaporative fuel treatment apparatus for an internal combustion engine according to claim 1, further comprising a mixing promoting portion that is provided between the communication portion and a connecting position at which the air supply passage is connected to the purge passage, and that promotes mixing of air introduced through the air supply passage with purge gas introduced to the purge passage.
 9. The evaporative fuel treatment apparatus for an internal combustion engine according to claim 8, wherein the purge passage meanders between the communication portion and the connecting position to form the mixing promoting portion.
 10. An evaporative fuel treatment apparatus for a multi-cylinder internal combustion engine that includes a pair of first and second banks respectively having a plurality of cylinders, comprising: a plurality of branch passages that respectively communicate with the plurality of cylinders in each bank; a plurality of throttle valves that are respectively provided in the plurality of branch passages in each bank, wherein each of the plurality of throttle valves is able to change the amount of air taken into a corresponding one of the plurality of cylinders; a balancing passage that communicates the plurality of branch passages with one another at positions downstream of the plurality of throttle valves in each bank; a communication portion that communicates the plurality of branch passages with one another at positions downstream of the plurality of throttle valves in the first bank; a purge passage that introduces purge gas, containing evaporative fuel generated in a fuel tank, to the communication portion in the first bank; an air supply passage that flows dilution air, which is used to dilute the evaporative fuel, into the purge passage in the first bank; a first flow rate changing portion that is provided in the air supply passage in the first bank and that is able to change the inflow of the dilution air; and a control unit that controls the opening degrees of the plurality of throttle valves in the second bank so that the amount of air taken into the second bank reduces by a differential flow rate, when an actual flow rate of dilution air that is changeable by the first flow rate changing portion is smaller than a predetermine flow rate of the dilution air used to reduce the concentration of the evaporative fuel below a predetermined concentration determined on the basis of an operating state of the internal combustion engine, wherein the differential flow rate is a difference between the predetermined flow rate and the actual flow rate.
 11. The evaporative fuel treatment apparatus for an internal combustion engine according to claim 10, wherein the flow passage area of the communication portion is smaller than the flow passage area of the balancing passage.
 12. The evaporative fuel treatment apparatus for an internal combustion engine according to claim 10, wherein the dilution air is air taken from an intake system upstream of the plurality of throttle valves.
 13. The evaporative fuel treatment apparatus for an internal combustion engine according to claim 10, wherein the control unit controls the opening degrees of the plurality of throttle valves in the second bank so that the amount of air taken into the second bank is determined so that combustion in the cylinders of the second bank is occurable and/or is larger than a lower limit intake air amount determined on the basis of an air-fuel ratio required in the second bank.
 14. An evaporative fuel treatment apparatus for a multi-cylinder internal combustion engine that includes a pair of first and second banks respectively having a plurality of cylinders, comprising: a plurality of branch passages that respectively communicate with the plurality of cylinders in each bank; a plurality of throttle valves that are respectively provided in the plurality of branch passages in each bank, wherein each of the plurality of throttle valves is able to change the amount of air taken into a corresponding one of the plurality of cylinders; a balancing passage that communicates the plurality of branch passages with one another at positions downstream of the plurality of throttle valves in each bank; a communication portion that communicates the plurality of branch passages with one another at positions downstream of the plurality of throttle valves in each bank; a purge passage that introduces purge gas, containing evaporative fuel generated in a fuel tank, to the communication portion in the first bank; a first air supply passage that flows dilution air, which is used to dilute the evaporative fuel, from an intake system upstream of the plurality of throttle valves into the purge passage in the first bank; a second air supply passage that flows upstream air, in the second bank, from an intake system upstream of the plurality of throttle valves into the communication portion of the second bank; a first flow rate changing portion that is provided in the first air supply passage in the first bank and that is able to change the inflow of the dilution air; a second flow rate changing portion that is provided in the second air supply passage in the second bank and that is able to change the inflow of the upstream air; and a control unit that controls the first flow rate changing portion such that the inflow of the dilution air is changed on the basis of a comparison between the concentration of the evaporative fuel and a predetermined concentration determined on the basis of an operating state of the internal combustion engine, and that controls the second flow rate changing portion such that the inflow of the dilution air is equalized to the inflow of the upstream air.
 15. The evaporative fuel treatment apparatus for an internal combustion engine according to claim 14, wherein the flow passage area of the communication portion is smaller than the flow passage area of the balancing passage.
 16. The evaporative fuel treatment apparatus for an internal combustion engine according to claim 1, further comprising: a second flow rate changing portion that changes the flow rate of the purge gas introduced from the fuel tank to the purge passage; a pressure measurement portion that measures a purge pressure at a position downstream of the second flow rate changing portion; and an estimation unit that estimates the purge pressure on the basis of an operating state of the internal combustion engine, wherein the control unit controls the first flow rate changing portion such that the inflow of the dilution air is maintained at a constant value, when the pressure measurement portion fails.
 17. The evaporative fuel treatment apparatus for an internal combustion engine according to claim 10, further comprising: a second flow rate changing portion that changes the flow rate of the purge gas introduced from the fuel tank to the purge passage; a pressure measurement portion that measures a purge pressure at a position downstream of the second flow rate changing portion; and an estimation unit that estimates the purge pressure on the basis of an operating state of the internal combustion engine, wherein the control unit controls the first flow rate changing portion such that the inflow of the dilution air is maintained at a constant value, when the pressure measurement portion fails.
 18. The evaporative fuel treatment apparatus for an internal combustion engine according to claim 14, further comprising: a second flow rate changing portion that changes the flow rate of the purge gas introduced from the fuel tank to the purge passage; a pressure measurement portion that measures a purge pressure at a position downstream of the second flow rate changing portion; and an estimation unit that estimates the purge pressure on the basis of an operating state of the internal combustion engine, wherein the control unit controls the first flow rate changing portion such that the inflow of the dilution air is maintained at a constant value, when the pressure measurement portion fails. 